Nurturing Bumblebees

Expert details on what makes magnificent bumblebees the perfect pollinators.

By Nancy Stamp

| Summer 2014

Like palace guards, the stamens stand in a circle around the pistil (with ovary within) forming a tube-like structure with an opening only at the exposed end. The pollen is firmly attached inside the anthers, capping the stamens. Something has to shake the stamens to release the pollen through pores in the anthers.Photo by Lew Stamp

A bumblebee investigates tomato flowers. Tomato flowers hang from the plant, with stamens bearing pollen pointing outward or downward; consequently the pollen doesn’t naturally fall on the stigma (which is surrounded by the stamens and is the gateway to the floral ovary).Photo by Lew Stamp

A bumblebee hangs onto the stamens with its jaws, holds its wings still, and vibrates its thorax muscles to shake pollen from the stamens.Photo by Lew Stamp

The pollen is captured by the bumblebee’s fur. By grooming in flight, she’ll store it on her hind legs (visible as a tan lump in this photo) and carry it home to her young. The flowers have tell-tale brown bruising on the stamens, left by earlier bumblebees grasping with their jaws while buzzing for pollen.Photo by Lew Stamp

Bumblebees, those big furry, burly bees — otherwise known as Bombus (the bumblebee genus, meaning "booming") — have been ignored by farmers until recently because bumblebee colonies produce very little honey surplus.

But the truth is, far from being useless annoyances, bumblebees are vital pollinators of native plants and as well as many crops. In North America, bumblebees are pollinators of clover, alfalfa, beans, blueberries and cranberries. Heirloom tomatoes and many cherry tomato cultivars — plus their relatives in the Solanaceae family, such as peppers and eggplant — need assistance from such buzz pollinators for pollination to occur. Bumblebees are the pollinator of choice.

What makes a bumblebee such a good pollinator? Bumblebees fly at lower temperatures than smaller bees because their large bodies covered with dense fur retain body heat better. Queens can fly at near-freezing temperatures and the smaller workers at 50 degrees Fahrenheit. A bumblebee averages 450 flower visits per hour — more than 7 flowers per minute — and works several hours more each day than a honey bee.

The bumblebee’s larger body size also means a longer tongue to obtain food from tubular flowers, and more hair to hold pollen dust. Bumblebees depend heavily on pollen for food and so will visit nectar-less flowers, such as tomato, that honey bees shun.

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Because bumblebees are such reliable pollinators, moving so much pollen around, more pollen is deposited on flowers. Up to point, this means more plant eggs are fertilized; in turn more seeds develop and the plants yield larger and juicier fruits. For a flower like the tomato, with a floral design making it difficult to self-pollinate, the bumblebee is the perfect match.

Bumblebees are also the champs of buzz pollination. Some flowers, such as the tomato, require quite fast vibration (equivalent to that of an electric toothbrush) to release their pollen. Honey bees are too small to create that, but bumblebees can. Bumblebees grab hold of the flowers with their jaws and then shiver their flight muscles while their wings are folded against their body. With no drag created by flapping wings, the bumblebees can vibrate twice as fast as honey bees, between 300-400 Hz. This causes the flower to shake enough to shoot pollen out, dusting the bumblebees. Then the bumblebees groom themselves of the pollen, even during flight, and store it on their hind legs in a spoon-like indentation where it is taken back to the nest to feed the bumblebee larvae. However, the bumblebees cannot groom themselves of all of the pollen. The pollen deposited at the base of the bee’s legs or between the thorax and abdomen is too difficult to groom, especially during flight. Consequently, some of the pollen dust ends up on the next flowers visited.

Bumblebee Lifestyle

The lifestyle of a bumblebee queen differs markedly from that of the honey bee. Unlike honey bees, bumblebee workers don’t hibernate over winter; instead, just the bumblebee queens hibernate. Then in the spring each queen searches for a nest site, investigating old rodent nests in grass tussocks or burrows in the ground.

Good bumblebee habitat is meadow with a variety of flowers available, or fields lined with thick grasses. A good nest site will already be supplied with hair, dry grass, feathers — insulation to make the nest warm and dry. Then the queen shapes the insulation into a tennis-ball size mass with a chamber inside, provisioned with a large lump of pollen and a wax honey pot.

Once the housework is done, the queen lays about 10-12 eggs on the pollen lump. Even in cool spring temperatures, she can incubate the eggs at a constant 86-90 degrees by shivering her flight muscles.

In a few days, the bumblebee larvae hatch and start eating the pollen. Resembling maggots, the bee babies are just eating machines. Consequently, the queen is busy both restocking the provisions and regulating nest temperature. Typically her work requires about 600 mg of sugar each day, which equals visiting as many as 6,000 flowers. It takes about 3-5 weeks to raise the baby bumblebees to adults.

Bumblebees at Risk

Unfortunately, just as honey bee populations are in decline, so are bumblebee species. This is due to combined effects of diseases, altered habitat, pesticide use, invasive species, and climate change.

In North America, comparison of historical distributions of eight Bombus species (>73,000 museum specimens) versus current distributions (U.S. survey with >16,000 specimens) showed that the range of four species — the American bumblebee (B. pensylvanicus), the yellow-banded bumblebee (B. terricola), the western bumblebee (B. occidentalis) and the rusty-patched bumblebee (B. affinis) — has shrunk by a whopping 23-87 percent.

Conserving Bumblebees

What then can be done now to mitigate bumblebee declines, thus ensure natural pollination? Wild bumblebees need a supply of pollen and nectar over their entire brood season, undisturbed nesting sites and hibernation sites. Nurturing bumblebee populations is similar to promoting other beneficial wildlife. Unmown margins around fields sown with a mixture of wildflowers, agricultural legumes and tussocky grasses attract the greatest abundance and diversity of bumblebee species. If a crop (such as tomato) doesn’t provide nectar, then bumblebee colonies need an additional nectar supply which can readily be provided by wildflowers.

Will nurturing wild bumblebees pay off? In California fields, wild bees have increased cherry tomato production, and wild bumblebees were a primary visitor when the tomato fields were within about 0.2 miles (or 1,000 feet) of natural habitat.

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Home gardeners can also help by providing shelters for bumblebees. A bumblebee colony needs a good nest site, plenty of food over the summer, and safety from predators and parasites. Queens will investigate dark, dry, warm holes, and make their home in a nest box if it suits their taste.

A nest box must be weather tight to keep the larvae warm and dry enough to deter mold and fungi growth. The box needs ventilation holes with fine screen to keep out ants, small drainage holes in the bottom, and nesting material such as upholsterer's cotton.

The queen needs nectar and pollen as soon as she emerges in the spring. That may require giving the spring-flowering weeds in the lawn, like dandelions, a break.

“What we see as weeds is dinner for a pollinator,” noted May Berenbaum, entomologist and coordinator of the University of Illinois' Bee Spotter program, which monitors bee populations by identifying bee species from photos throughout Illinois. Talking with the St. Louis Beacon, she emphasized, “A perfectly green, manicured lawn is a biological desert for pollinators.”

Furthermore, often the flowers in home gardens are introduced species, or horticulturally modified in ways that don’t match the native bees’ needs. For instance, cultivars may be selected for flowers with double the number of petals — visually a super-sized flower. But extra parts may interfere with the pollinator’s access to nectar and pollen. More petals may also have a production cost in the form of less nectar and pollen. In general, such flowers reduce the diversity of insect visitors, and in turn, seed production is likely less. That gives home gardeners, especially the tomato lovers, another reason to think twice about when and what flowers are available to pollinators.

For more information about bumblebees, the Xerces Society for Invertebrate Conservation recently published a free document entitled “Conserving Bumble Bees: Guidelines for Creating and Managing Habitat for America’s Declining Pollinators,” which provides color identification guides of bumblebees, lists of flowers by season and region that attract bumblebees, and nest box instructions. To access this document, visit: www.Xerces.org/bumblebees/guidelines/

Nancy Stamp is a biologist at Binghamton University-State University of New York, researching the tomato and insects that use it.

Lew Stamp is a freelance professional photographer in Akron, Ohio, and is a dedicated gardener.

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